It is proposed to continue purification of the RNA-proteinaceous complexes present in phenol extracts of mammalian thyroids and to study their chemistry and biological functions. Yeast will be examined for similar complexes. Complexes Ia and Ib from porcine thyroid will be used to study the chemistry of the complexes. The nucleotide, amino acid and carbohydrate content and the determination of molecular weights will be undertaken. A biological role for Ia will be sought using in vitro aminoacylation and protein biosynthesizing systems. The amino acid accepter activity of porcine thyroid complex II will be studied by extending results, obtained using Tyr-complex II, to other amino acids that are incorporated into the complex by comparisons between aminoacylated tRNA and complex. Tyrosyl-tRNA synthetase will be isolated and its interaction with purified Tyr-tRNA and Tyr-complex II contrasted. The stimulatory and inhibitory effects that complex II exerts on the aminoacylation of isoleucyl- and aspartyl-tRNA will be pursued by studying the interaction of complex II with purified isoleucyl- and aspartyl-tRNA synthetases.